FORCES AND NEWTONS LAWS by ewghwehws

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									FORCES AND NEWTON’S
              LAWS


     2.1-2.2
                           What is a force?

               WHAT IS A FORCE?

            A force is any push or pull

           WHAT DOES FORCE DO?

A force changes the velocity of an object, because it
          changes its speed and/or direction
        Why is force hard to define?

• It is something that you take for granted that
  exists – although you know about it, you
  have never had to be specific about it
• Also: there are many forces that are
  INVISIBLE
• If you push/pull on an object, you know
  where it comes from because you can see it
  happening
• But there are some that you can’t see
       Examples of “invisible” forces

• Gravity
  • Objects falling towards the earth are changing
    their speed – can you see what causes them to do
    this?
• Friction
  • Rolling objects that are pushed come to a stop –
    but can you see what causes them to do this?
                              Enter Newton

• Based on these basic observations, a scientist
  named Sir Issac Newton came up with 3 laws
  that describe the behaviour of forces
• These are known as NEWTON’S LAWS OF
  MOTION
    Newton’s first law: ask yourself this

•  Collisions are a good way to start thinking about
   Newton’s first law
• When two objects collide – one object applies a
   force onto the other
  • When a car collides into a wall: what happens
     to the car and the wall?
  • When a truck collides into a car, what happens
     to the truck and the car?
• http://www.consumerreports.org/cro/resour
  ces/streaming/crashtestpopups/ToyotaSolar
  a/TOYOTA-1.html
                            What happens?

• Between the wall and the car:
  • The wall doesn’t move
  • The car rebounds


• Between the truck and the car:
  • The truck moves forward
  • The car moves forward
                                      Why?

• IS THE CAR APPLYING A FORCE
  ONTO THE WALL?
• Yes
• WHY DOESN’T THE WALL MOVE?
• Because the wall is rooted to the ground – it
  is hard to move the wall
                                       Why?

• WHY DOES THE TRUCK MOVE
  FORWARD EVEN AFTER SLAMMING
  INTO THE CAR?
• Because the truck has a heavier mass than
  the car – and therefore applies a large force
  that can cause the car to move without
  stopping itself
                             In both cases….

• The force that is applied to the object will only
  move the object if the force is great enough
• OBJECTS DON’T MOVE OR CHANGE
  THEIR MOVEMENT UNLESS THEY ARE
  FORCED TO!
• The amount of force required depends on the
  object’s:
  • Mass
  • Friction keeping it in place
            What did Newton call this?

• INERTIA: the “desire” of an object to maintain its line
  of motion

Only a force great enough to overcome an object’s
 INERTIA will cause it to change its direction

That’s why:
• The wall stayed still
• The truck kept moving – the car didn’t apply a great
  enough force to overcome its inertia to make it stop
   NEWTON’S FIRST LAW: LAW
                 OF INERTIA

An object at rest tends to stay at rest and an
 object in motion tends to stay in motion with
 the same speed and in the same direction
 unless acted upon by an unbalanced force.
                            What the heck?

• An object at rest tends   = An object won’t move
  to stay at rest             on its own
• and an object in          =something moving
  motion tends to stay in     won’t change speed or
  motion with the same        direction
  speed and in the same
  direction                 = unless a force comes
• unless acted upon by        along to change it
  an unbalanced force.
     What is an unbalanced force?

• Unbalanced = not equal
• For example:
• If one person pushes in one direction on an
  object:                    The object moves
                            because there is only
                            one force moving in
                            one direction – IT’S
                            NOT BALANCED
        What happens when forces are
                          balanced?

• If two people push with the exact same
  force but in different directions on the same
  object:
                             The object won’t move
                             because the two forces
                             BALANCE EACH
                             OTHER AND
                             CANCEL EACH
                             OTHER OUT!
                    Newton’s second law

• Physics is still about numbers
• So how do we put numbers into studying forces?
• We have to be able to MEASURE force somehow
  – which means numbers
• So: we know force causes an object to move
• How do we calculate movement? (We just studied
  this)
• DISPLACEMENT, VELOCITY OR
  ACCELERATION
• WHICH ONE IS BEST?
                    DISPLACEMENT

• Difficult to tell – remember that just
  because an object travelled far doesn’t mean
  that it was pushed/pulled hard
• You have no idea how long it took to do it
• If you have no idea how much time was
  involved – then it could have been going
  slow (little force) or fast (more force)
                              VELOCITY

• Velocity overcomes the problem with
  distance – it tells you both time and distance
• But – objects can have a speed even though
  nothing is pushing it at the moment
                  SLIDING OBJECTS
                  HAVE SPEED BUT
                  NO APPARENT
                  FORCE APPLIED TO
                  THEM – SO SPEED
                  IS NOT RELIABLE
THAT LEAVES ACCELERATION

• Acceleration is the best way to measure
  force
• Acceleration describes changes in speed and
  direction – both of which are important to
  consider when studying forces
• Slowing down, speeding up or changing
  direction ALL REQUIRE FORCES
       So what is force dependent on?

• Think about accelerating a truck vs. a car
• WHICH ONE ACCELERATES FASTER?
• A car
• WHICH ONE HAS A STRONGER ENGINE?
• A truck
• LARGER ENGINE: DOES IT EQUAL MORE FORCE
  OR LESS FORCE?
• Larger engines generate more force to push a vehicle
• WHY DOES A TRUCK ACCELERATE SLOWER
  EVEN THOUGH THE ENGINE IS LARGER?
• Because the truck is heavier
So acceleration of an object depends
                                 on:

• The amount of force
• The mass of the object

Newton’s second law:
The acceleration of an object as produced by a net
  force is directly proportional to the magnitude of
  the net force, in the same direction as the net
  force, and inversely proportional to the mass of
  the object.
                                     What the heck?
• The acceleration of an object as   =acceleration created by a net
  produced by a net force               force
• is directly proportional to the    =increases if net or unbalanced
  magnitude of the net force            force increases, decreases if it
                                        decreases
• in the same direction as the net   =if you push/pull left, object will
  force,                                accelerate towards the left -
                                        therefore, an object will
                                        accelerate in the same direction
                                        as net force
• and inversely proportional to      =acceleration increases if mass
  the mass of the object.               decreases, decreases if mass
                                        increases
                                        Again….

•   If force increases, acceleration increases
•   The stronger the push, the faster it increases speed
•   If force decreases, acceleration decreases
•   The weaker the push, the slower it increases speed
•   If mass increases, acceleeration decreases
•   The heavier the object, the slower it increases speed
•   If mass decreases, acceleration increases
•   The lighter the object, the faster it increases speed
                        Now the math..

• Mathematically:


             Fnet = ma
• Fnet = net/unbalanced force acting on an
  object in N (Newtons)
• m = mass of object in kg
• a = acceleration of object in m/s2
                               Weight vs. mass

• MASS: the amount of mass in an object measured in g or kg
• WEIGHT: the force of gravity acting on an object
• Every object on earth accelerates towards the earth at 9.8
  m/s2
• Therefore, every object’s weight or force of gravity acting
  on it is based on Newton’s second law
• Since: F = ma
• And: a = g
• Then force of gravity (Fg):            Fg = mg
          The third and funniest law

• We know that objects only move if a force
  is applied on them – by the 1st and 2nd law
• But there are some cases where this is
  difficult to see
                  Pushing off a wall?

• Think about someone wearing rollerblades
  that pushes off the wall
• What happens?
If the child pushes on the wall
 She moves backwards


Does this make sense when you follow
Newton’s second law?
                              Direction of
                              acceleration of
No! According to              girl
the 2nd law, the
direction of force
and acceleration has
to be the same
                                                Direction of
                                                force applied
                                                by girl
Based on this fact, Newton came up with the idea
that in order for the girl to be moving backwards –
THE WALL MUST BE PUSHING ON HER!

This is the idea
behind Newton’s            Direction of
3rd law: that              acceleration of
anything that is           girl

pushed pushes
back

                                             Direction of
                                             force applied on
                                             girl by the wall
                                  That’s weird

• Does that sound strange?
• If you think about it – if you apply a force, there has
  to be contact between two surfaces
• When two objects touch, force is exchanged
  between the two
• Imagine holding two pieces of paper in both hands,
  and pushing against each other – they both bend!
• And the force exchanged between the two are equal
    Then why doesn’t the wall move?

• This goes back to understanding Newton’s
  first law – and also the collision between
  the car and the wall
• The girl pushes with a certain amount of
  force against the wall – but because the wall
  has a lot of inertia, the amount of force is
  not great enough to move it
    Then why does the girl move?

• The girl has less inertia than the wall
• She is on wheels – which decreases the
  amount of force needed to move something
• So even a little force – the amount that she
  is pushing off the wall – will cause her to
  move
Direction of
acceleration of
                  Direction of
girl
                  force applied on
                  girl by the wall



                    Direction of
                    force applied
                    by girl
                   Newton’s third law

• For every action, there is an equal and
  opposite reaction.
                         What the heck?

• For every action       • For every push or pull
                           on an object
• There is an opposite   • The object
  and equal reaction       pushes/pulls back in
                           the opposite direction
                           on whatever it was
                           that pushed/pulled on
                           it
    Newton’s      3 rd   law explains why…

• You don’t fall through the floor when you stand on
  it!
• Think about it: when you stand on something, you
  are essentially pushing down on it because gravity
  pulls down on you, and that pushes on the floor
• The reason why you don’t push through the floor is
  because the floor pushes back on you
• Since the forces are balanced – you stay on the
  ground
If force of gravity is greater than the force of the floor…
If the force of the floor is greater than the force of gravity…


If both are equal…the boy stays on the ground!

 Force of floor
 pushing back
 up on boy



                            Force of gravity pushing on floor
                           Normal force

• Normal force is the force that a surface
  pushes back on an object that bears weight
  on it
• For example: your textbook sitting on your
  desk pushes down on your desk, and the
  desk pushes back on the book
• The force pushing back on the book is
  known as the normal force (FN)
              Communicating forces

• Free body diagrams are used to show all the
  forces acting on an object
• You’ve seen how they have worked already
  in describing the boy on the ground or the
  girl pushing on the wall
          Rules for free body diagrams

• The object you are drawing is represented as a box
• Arrows are used to represent each force on the box
• The tail end of the arrows attach to the box
• The arrows move in the direction of the force acting
  on the object
• Don’t forget to add forces that you can’t see like
  friction, gravity, and normal force
Draw a free body diagram of the forces on the
cart


                                 Normal force
                                 on cart

                                                Force of man
                   Force of                     pushing cart
                   friction on
                   cart


 Notice that you don’t
 have to worry about              Force of
 drawing the man in the           gravity on
 free body diagram                cart
        Draw the free body diagram of the
                forces on the garbage bag
                                          Force applied by the boy
Notice: since the object is hanging       holding the bag
and is not touching the floor             (TENSION)
there is no normal force since
there is no surface contacting the bag!




                                     Force of gravity on the bag

								
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